Among three-layer membranes (pia mater, cranial arachnoid mater, and cranial dura mater) that are present in the cranial bone and protect the brain parenchyma, the cranial dura mater is hardest and exists as the outermost layer of the three aforementioned layers. Thus, the cranial dura mater is also considered to be the inner periosteum of the cranial bone. When neurosurgical operations are conducted, the cranial dura mater must be excised in many cases, thereby resulting in the absence of the cranial dura mater. In addition, there may also be cases where spontaneous contraction of the cranial dura mater per se makes the primary suture difficult. If the operative site were sutured without closing the cranial dura mater, it would cause serious complications. For example, it could cause a leakage of spinal fluid, which may result in development of an intracranial infection, or it could cause the adhesion of the brain parenchyma to the bone or subcutaneous tissues, which may lead to local nervous symptoms or become a focal point of a seizure. Thus, when the operative site is sutured, a strict suture is required, so as to avoid the formation of gaps on the cranial dura mater. Accordingly, in a case where a part of the cranial dura mater is lost, or where the primary suture becomes difficult, it is necessary to completely suture the operative site using a certain supply material, so as to prevent the formation of gaps.
The type of a supply material that is used to compensate for the absence of the cranial dura mater has been an important issue for a long period of time, and has been discussed by neurosurgeons. Artificial materials were initially used for a while, but they have been problematic in terms of biocompatibility, handlability, etc. Thus, such artificial materials were soon abandoned. Auto fascia has most widely been used from the initial stage to date. However, such auto fascia is also problematic in that the original fascia is lost from the excised site, and that it easily adheres to the brain. Dry human dura mater is a dura mater supply material, which is produced by treating the dura mater collected from the dead body with radioactive rays. This material was the best among those that had ever been produced. However, the possibility emerged that prions reportedly responsible for Creutzfeldt-Jakob disease exist in the dura mater. It was then reported that a patient became affected with Creutzfeldt-Jakob disease through the dry human dura mater. In consequence, the use of dry human dura mater was completely prohibited in 1998.
At present, ePTFE (expanded polytetra-fluoroethylene) certified by the Ministry of Health, Labor and Welfare is the only material that can be used as a dura mater supply material other than the aforementioned auto fascia. Since ePTFE is a polymeric material, it has no adhesive properties to living bodies. This is advantageous in that the material does not adhere to the brain. On the other hand, since ePTFE has poor contractility, the spinal fluid leaks through pin holes. Thus, it is necessary to suture the operative site using a special surgical suture. Moreover, since ePTFE does not have adhesive properties to living bodies, spinal fluid leakage occurs even from gaps on a sutured surface. Furthermore, since ePTFE does not have adhesive properties also to peripheral tissues, there is a high possibility that it functions only as a simple skeletal material. A large number of attempts have been made to utilize such ePTFE to as great an extent. All of these attempts relate to a technique of using ePTFE as a skeletal material for the subsequent formation of fibrous tissues around the ePTFE.
As a method of treating the surface of an artificial material with ions, a method involving plasma treatment has been known (Japanese Patent Application No. 10-302170). This method comprises improving adhesive properties by modifying the surface of the material. A plasma-treated layer obtained by the plasma treatment method is unstable in living bodies, and it has a risk of decomposing or peeling off over time. In living bodies, it is necessary to maintain a stable cell adhesion layer over a long period of time. When the aforementioned plasma treatment method is particularly applied to the artificial dura mater, although the material adheres to the contact surface of the cranial bone at the initial stage, it has a risk of peeling off after a long period of time.
It has also been reported that the surface layer of an in vivo embedded material is modified using ions with higher energy than the ions used in the plasma treatment, so as to enhance antibacterial properties (Japanese Patent Application No. 5-148994). The main purpose of this method is to reduce the infectivity of the embedded material.